When using a semiconductor single crystal wafer, e.g., a silicon wafer to fabricate a device, many processes lie between a wafer processing process to a device forming process, and there is a heat treatment process as one of such processes. The heat treatment process is an important process that is performed for the purpose of forming a defect-free layer in a surface layer of a wafer, gettering, crystallization, formation of an oxide film, or diffusion of an impurity.
As a diffusion furnace (an oxidation/diffusion device) utilized in such a heat treatment process, e.g., oxidation or diffusion of an impurity, a vertical heat treatment furnace in which a heat treatment is performed with many wafers being horizontally supported at predetermined intervals is mainly used with an increase in diameter of wafers. Further, when using the vertical heat treatment furnace to perform a heat treatment to wafers, a vertical heat treatment boat (which may be simply referred to as a boat hereinafter in some cases) that is used to set many wafers is utilized.
FIG. 4 shows an outline of a conventional general vertical heat treatment boat 210. A pair of plate-like members (which will be also referred to as coupling members or a top plate and a bottom plate) 216 are coupled to both end portions of each of four support rods 214. Many slits 211 are formed on each support rod 214, and a convex portion between the respective slits 211 functions as a support portion 212 for wafers. When performing a heat treatment with respect to a treatment target wafer (which may be simply referred to as a wafer hereinafter), as shown in a plan view of FIG. 5(A) and a front view of FIG. 5(B), each wafer W is horizontally supported by mounting an outer peripheral portion of the wafer W on the support portions 212 formed at the same height of each support rod 214.
FIG. 6 is a schematic view showing an example of a vertical heat treatment furnace. Many wafers W are horizontally supported by a heat treatment boat 210 installed in a reaction chamber 222 of a vertical heat treatment furnace 220. At the time of a heat treatment, the wafers W are heated by a heater 224 provided around the reaction chamber 222. During the heat treatment, a gas is led into the reaction chamber 222 through a gas introducing pipe 226, flows from an upper side toward a lower side, and is discharged to the outside from a gas exhaust pipe 228. Although the gas to be utilized varies depending on a purpose of the heat treatment, H2, N2, O2, or Ar is mainly used, for example. In case of impurity diffusion, each of these gases is also used as a carrier gas for an impurity compound gas.
Various kinds of shapes are adopted for the wafer support portions 212 in the vertical heat treatment boat 210, and each of FIGS. 7(A) and (B) shows an example. (A) shows an example in which concave slits (grooves) 211 are provided in each support rod 214 having a semi-cylindrical shape to form semi-circular support portions 212. On the other hand, (B) shows an example in which concave slits 211 are provided in each wide prismatic support rod 215 to form rectangular support portions 213 in order to support each wafer W at a position closer to the center thereof than (A). Besides, a slit shape may be a circular shape or a hook-like shape.
Further, rather than directly supporting the wafers W by such support portions 212 or 213, there is also a conformation that wafer support jigs are detachably disposed to the respective support portions 212 or 213 and the wafers W are mounted on and supported by the wafer support jigs.
A contact surface (a support surface) with respect to the wafer of the wafer support jig (or a support portion) that directly comes into contact with and supports such a treatment target wafer will now be described.
Japanese Unexamined Patent Publication (Kokai) No. H09-283455 has a description that a factor of stress concentration due to a weight of each wafer itself is removed by setting surface roughness Ra of the support surface to 0.5 μm or below to suppress slip.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2000-269150 has a description that the surface roughness Ra of the support surface is set to 0.3 μm or above since each wafer and the wafer support jig partially adhering to each other when the support surface is too smooth.
As explained above, to achieve both preventing stress concentration due to a weight of the wafer itself and preventing the wafer/wafer support jig from adhering, the surface roughness must be adjusted to fall within an optimum range, and the surface roughness is adjusted by a method such as a grinding process or a sandblast process.
However, even if a wafer support surface adjusted by such a method is used, slip cannot be suppressed when a thermal stress factor is increased by raising a temperature increase/temperature decrease rate at the time of a heat treatment or reducing a boat pitch. It can be considered that slip cannot be suppressed because of an influence of thermal deformation of the wafers or the wafer support jigs. When there is friction or adhesion between the wafers and the wafer support jigs, a stress that avoids thermal deformation is produced at the time of an increase or a decrease in temperature. It can be considered that, when a temperature increase/decrease rate is raised or when a boat groove pitch is reduced, alleviation of deformation due to such as slide of the wafers on the wafer support jigs caused by an increase in thermal deformation rate is no longer sufficient, and hence the stress increases, resulting in slip.
To reduce this stress, Japanese Patent Publication No. 3328763 discloses a method of reducing friction between each wafer and each wafer support jig by using a roller for the wafer support jig. However, there is a problem that accurately processing a concave portion in which the roller is mounted is difficult and a manufacturing cost for a boat is increased.